I bought a used unlocked 3950 cortex and was frustrated trying to get it to load to my truck. i tried many times and got G-3F general errors. here's what worked in case someone else is trying the same thing.

1. go to the superchips site and start your file and hook up the cortex to make sure it talks. don't worry about the load yet.
2. take it to the truck and go to additional features/identify vehicle in the menu and do what it says. check if your truck's vin is in there after it is done.
3. take it back to the computer, hook it up , and let it download. if you ask for the other packages like tpms or datalogging they will go in now.
4. after it is done the download, take it to the truck and it should do what it is supposed to.

it seems like the maps won't go in unless the vehicle has been identified, it has the old owner's truck still in as a vin, but not locked. hope this helps someone.